1. Field of the Invention
The present invention relates to a creeping discharge-prevention means or a partial discharge-preventions means for a stator coil end used for a rotary electric machine.
2. Description of the Prior Art
In general, a stator coil for a high voltage rotary electric machine is insulated from the ground by using an insulator made of organic or inorganic insulation material or a mixture thereof. An electric conductive paint is coated on the surface of the insulator on a linear part of the coil disposed in a slot of the iron core of a stator in a length the same or longer than the length of the iron core.
When the stator coil is disposed in the iron core of a stator having ground potential, the surface of the iron core and the coil of the stator has a potential higher than the ground potential for a potential drop caused by the resistance of the conductive layer coated on the surface of the coil.
However the potential difference is quite small so that the potential of the conductive coated layer is substantially the same as ground potential.
When the voltage is applied to the conductive wire of the coil of a stator, the conductive wire of the coil and the grounding electrode of the conductive coated layer form a pair of electrodes so that the line of electric force is crossed to the surface of the insulator at the coil end. The electrostatic capacity between the pair of electrodes is high. Accordingly, the potential gradient of the surface of the insulator in the creeping direction of the coil is high. The potential gradient is especially high at the end of the grounding electrode and a partial discharge or a creeping discharge is easily caused at the part having a high potential gradient.
The potential gradient increases depending upon an increase of the voltage applied to the coil. Accordingly, the discharge is easily caused from a coil having higher rated voltage then from a coil having a lower rated voltage. When a stator coil for a high voltage rotary electric machine is manufactured, the dielectric strength test of Japanese Electrotechnical Committee -- 114 (1965) is carried out under the Rule. For example, in the case that the rated capacity of the rotary electric machine is 10,000 KW or higher and the rated voltage E is higher than 6,000 V, the coil should be satisfactory in the dielectric strength test of the insulator layer to the charging part while applying the test voltage Et for 1 minute, wherein Et = 2E + 3,000 V (1)
(effective value)
In a manufacturing factory of the rotary electric machine, it is usual to carry out the dielectric strength test by applying a test voltage of about 1.1 .about.2.0 Et in the process of manufacture so as to confirm the reliability of the insulator part of the rotary electric machines. Accordingly, in tests of rotary electric machines having rated voltage of 20 KV to 20 KV, the test voltage is quite high. Thus, the potential gradient is remarkably high because of the high voltage applied to the coil. Accordingly, sometimes it is difficult to carry out the dielectric strength test because of the creeping discharge during application of a voltage lower than the test voltage.
The purpose of the present invention is to provide simple and effective circuitry for preventing a partial discharge or a creeping discharge of the coil end of the stator having a high rated voltage by decreasing the potential gradient of the surface of the stator coil end for the rotary electric machine in the creeping direction.
FIG. 1 is a partial sectional view of a conventional stator coil end. In FIG. 1, the reference numeral 1 designates a conductive wire of a coil; 2 designates an insulator; 3 designates a grounding electrode such as a conductive coated layer of paint.
When a high voltage is applied to the conductive wire 1 of the coil, a pair of electrodes are formed by the conductive wire 1 and the grounding electrode 3 whereby the potential gradient at the end of the grounding electrode 3 in the creeping direction of the coil is high. In order to prevent the increase of potential gradient, it has been proposed to coat an electrical stress grading paint 4 on the surface of the insulator 2 including the end of the grounding electrode 3.
The electrical stress grading coated layer 4 should have a non-linear characteristic of resistance of the coated film to the field to impart resistance as an insulator in a low field intensity and resistance as a semiconductor in a high field intensity.
When the electrical stress grading coated layer having nonlinear resistance is disposed at the stator coil end, and the voltage V (effective value) is applied to the conductive wire 1, the current is passed through the electrical stress grading coated layer 4 depending upon the charge current to cause power consumption. The relation of the voltage V to the maximum power consumption Wm in the electrical stress grading coated layer 4 is given by the equation EQU Wm = 1.3 .omega.c V.sup.2 [W/m.sup.2 ] (2)
wherein .omega. = 2 .pi.f;
f represents the frequency of the power source [Hz]; and c represents the electrostatic capacity per unit area of an insulator of the coil. [F/m.sup.2 ].
The power consumption is proportional to the square of the voltage applied. The power consumption caused by Joule heat as the basic property of the commercial electrical stress grading coated layer at the breakdown is about 1 .about.2 .times. 10.sup.4 [W/m.sup.2 ]. Accordingly, when the field intensity reducing coated film is disposed at the stator coil end, the power consumption Wm of Equation (2) is limited and the voltage V applied is limited. That is, the electrical stress grading coated layer 4 is burned by the Joule heat of Equation (2) in a coil having a high rated voltage. The burning of the electrical stress grading layer 4 causes the deterioration of the insulation of the insulator 2 or the loss of a non-linear characteristic of resistance of the electrical stress grading coated layer 4 causing a linear characteristic of resistance to act as an insulator.
The loss of the non-linear characteristic of resistance of the electrical stress grading coated layer 4 yields resistance as an insulator in a high field intensity. Accordingly, the effect of the electrical stress grading coated layer 4 is lost and the potential gradient at the end of the ground electrode 3 is increased to cause the creeping discharge. Accordingly, the applicable voltage V under the use of the electrical stress grading coated layer is limited.
In order to overcome the disadvantage from the use of the electrical stress grading coated layer, the voltage applied under the use of the coated layer can be increased by increasing the thickness of the insulator 2. However, it is not satisfactorily effective as described below. The method utilized is to decrease the power consumption Wm by decreasing the electrostatic capacity c of Equation (2). It is known that the applicable voltage can be increased by decreasing the power consumption Wm as an effect of the thickness of the insulator.
In the conventional method for reducing the creeping field intensity of the surface of a coil by disposing voltage dividing electrodes into the insulator 2 at the coil end (c voltage dividing method), it is necessary to dispose certain voltage dividing electrodes into the insulator 2. It is quite difficult to control the positions of the electrodes and accordingly the operation efficiency in the manufacture of the coil is disadvantageously low. Moreover, as the electrodes are disposed into the insulator 2, the substantial disadvantage of deterioration of insulation of the insulator 2 to the ground has been found to exist. As illustrated above, the effect of the field of intensity reducing coated film is limited. Accordingly, the conventional method can not be applied for a stator coil having a high rated voltage. Further, the method for increasing the thickness of an insulator does not impart a desirable effect and the c voltage dividing method has the above-mentioned disadvantages.